1. Field of the Invention
The present invention relates to an aqueous gel comprising carrageenan, to a gel-forming composition and a dry composition to constitute such aqueous gel, and to a composite containing same.
2. The Related Art
For various applications there is a need for aqueous gels that have a fairly high gel strength and a relatively low melting point. For example, for food-products it is often desirable that the gel melts in the mouth to release the flavour. The gelling agent commonly used to meet these demands is gelatin. With gelatin, aqueous gels can be made that usually have a melting point of about 30.degree.-35.degree. C. Gelatin can be used to prepare very weak gels; but, alternatively, rather firm gels can suitably be obtained with gelatin as well. For example, the gel strength of aqueous gelatin gels, as indicated by the yield stress at 5.degree. C., may range from about 0.1 to above 200 kN/m.sup.2.
However, the use of gelatin is not without disadvantages. For example, gelatin is a rather expensive gelling agent. The gelatin gel sets rather slowly, which can be undesirable. Furthermore, the use of gelatin in food-products can be undesirable for e.g. Jews and Moslems, because it is prepared from pigskin. Despite these disadvantages, however, a commercially acceptable gelling agent for the preparation of fairly strong low melting aqueous gels, that can be used instead of gelatin, has never been found.
Similarly, for aqueous gel cosmetic and pharmaceutical products intended for topical application to the skin, the mucosae or the teeth, it is also desirable that the gel is firm at room temperature yet softens at or near body temperature to facilitate its cosmetic or pharmaceutical benefit.
Kappa- and iota-carrageenan are commonly used polysaccharide gelling agents. With carrageenan gelling agent compositions as commonly available, fairly firm gels can conveniently be obtained. However, such gels have high melting points, e.g. from about 50.degree. C. to about 70.degree. C. or even higher.
The gel melting point is the temperature at which the gel network structure disappears if the gel is heated slowly.
However, in practice it is difficult to determine this temperature accurately and reliably and it is, therefore, common practice to refer to the closely related transition midpoint temperature Tm, which is determined by means of optical rotation dispersion (ORD). The Tm-value of carrageenan gels commonly lies within the experimental error range of the gel melting temperature. However, the Tm-determination via ORD depends on the transition from the ordered to the disordered state. The ordered state may be a gel, but this need not be so. Aqueous carrageenan systems may also occur in the so-called double helix state. In this state, it is believed, pairs of polymer molecules have formed, presumably in a double helix conformation, which is indicated by the ORD measurement as an ordered state, but macroscopically the system is still a pourable liquid, because no wide ranging three dimensional network has formed. Thus, for carrageenan gels the Tm-value usually does not differ substantially from the gel melting point. However, the fact that the midpoint temperature for the order to disorder transition, Tm, can be determined does not imply that the system at the lower temperature is a gel; the ordered state may also be the non-gelled double-helix state.
Although in practice the Tm-value of a carrageenan gel is often used interchangeably with the temperature at which the gel melts, which is correct for the majority of carrageenan gels, substantial differences between the temperature at which a gel comprising carrageenan melts and the Tm-value of that gel, may occur. If for example, in a carrageenan gel composition that has a Tm-value and a gel melting point of about 50.degree. C., locust bean gum is incorporated, this can cause an increase of the temperature at which the gel melts to e.g. about 55.degree. C. or 60.degree. C. without changing the Tm-value of the order to disorder transition. For the large majority of carrageenan gels, however, the gel melting point does not differ substantially from the Tm-value.
It is well known that the properties of aqueous carrageenan systems may vary substantially in dependence of the type and quantity of ions contained in the system.
In U.S. Pat. No. 2,864,706 it is described that the elasticity of carrageenan gels can be improved by incorporating the potassium salt of a sequestering agent, which serves to chemically bind the ions to which the brittleness of conventional carrageenan gels is attributed and to supply an abundance of potassium to enhance gelation.
U.S. Pat. No. 3,962,482 is directed to clear, elastic, water gels and gel-forming compositions that include a potassium salt and a potassium sensitive carrageenan. The gels and the gel-forming compositions are characterized by essentially complete freedom from polyvalent metal ions.
In U.S. Pat. No. 3,956,173 it is described that kappa-carrageenan becomes readily soluble in cold water when it is converted to the sodium salt form. The solubility is said to be considerably reduced by the presence of potassium salts. U.S. Pat. No. 3,956,173 teaches to encapsulate a potassium salt, such as e.g. potassium chloride, with a water-soluble hydroxypropyl cellulose, and to blend it in dry form with sodium kappa-carrageenan, before addition of the mixture to water to effect gelling.
U.S. Pat. No. 4,307,124 describes the preparation of a carrageenan gel with the use of a composition that is swellable in cold tap water and gels at room temperature. The composition comprises kappa-carrageenan containing less than 5 wt. % of each of potassium and calcium cations, potassium bitartrate, cold soluble locust bean gum and a sodium salt of a sequestering agent. The sequestering agent is present in an amount sufficient to sequester substantially all of the polyvalent cations present when the composition is dissolved in cold tap water.
Although the above mentioned publications are directed to various objectives, they have in common that the disclosed gels have high melting points. The mid point temperature of the order to disorder transition, Tm, is above about 45.degree. or 50.degree. C. and for the majority of the disclosed gel compositions it is substantially higher.
U.S. Pat. No. 4,276,320 discloses gels with relatively low melting points, e.g. at about body temperature. The gel-forming compositions comprise kappa-carrageenan, potassium in an ionizable form in an amount sufficient to provide the sol (after solution of the composition in an aqueous medium) with a potassium ion content of from about 200 to about 800 ppm, and a sodium salt of a sequestering agent in an amount sufficient to sequester substantially all of the polyvalent cations present in the sol. The gels disclosed in U.S. Pat. No. 4,276,320 have Tm-values well below 45.degree. C. However, the gels are extremely weak. At 1.degree. C., the yield stress is below the measurement range of a conventional Instron.RTM. Tester. At refrigerator temperature, the gels do not retain their shape but are still pourable.